https://github.com/mongodb/specifications/blob/master/source/transactions/transactions.rst#unknowntransactioncommitresult says:

> The driver MUST add the "UnknownTransactionCommitResult" error label when commitTransaction fails with a server selection error, network error, retryable writes error, MaxTimeMSExpired error, or write concern failed / timeout.

As far as I can tell this label is not added to our server selection errors.

As part of this work the MaxTimeMSExpired case should be considered (it may be covered by a separate ticket & spec tests).